DIURNAL AND SEASONAL REDOX CHANGES IN LAKE SEDIMENTS: IMPLICATIONS FOR NUTRIENT FLUX AND CYANOBACTERIAL BLOOMS

Nutrient mobility across the sediment-water interface (SWI) in Missisquoi Bay, a shallow (<15m) bay in Lake Champlain, is largely controlled by redox changes exhibiting diel and seasonal variability that can significantly impact nutrient fluxes and associated cyanobacterial activity (resulting in harmful algal blooms). We investigated changes in sediments, sediment porewaters, and the overlying water column over two consecutive summers (2007, which did not exhibit a bloom, and 2008, which did exhibit a bloom), with detailed monitoring of diel changes at each sampling time. Sediment cores were profiled using voltammetric microelectrodes to characterize porewater redox chemistry (O₂, Mn²⁺, Fe²⁺, Fe³⁺, HS^-, FeS_(aq)), profiled for pH, and partitioned and extracted for nutrients and associated cations (Fe, Mn, Ca, Al). Redox profiles of cores show redox conditions become more reducing as the season progresses, with the most strongly reducing conditions (no O₂ penetration into sediment, measureable Mn²⁺ and Fe²⁺ in the overlying water) present during peak bloom conditions and at dawn. In sediments, reactive P (RP) is strongly correlated to reactive Fe, indicating the mobility of a large portion (30-40%) of the P pool in the sediment is associated with iron oxyhydroxide minerals. RP concentrations in the top sediments increased through 2007 but decreased through 2008, suggesting a portion of the sediment-bound P was released into the water column in 2008. Porewater ammonia levels in sediment cores also increased as conditions in the sediments became more reducing..

Redox conditions were also measured continually over 24-hour periods using in-situ voltammetric electrodes positioned at the SWI for each sampling time over the 2007 and 2008 seasons. Somewhat surprisingly, even at peak daylight conditions in the presence of a cyanobacterial bloom in 15 feet of water, redox conditions at the sediment-water interface are more reduced in the presence of a bloom than in the absence of a bloom. Redox conditions exhibited significant changes over diel cycles, with nighttime conditions becoming most reducing. As P fluxes are sensitive to a thin layer of iron oxyhydroxides at the sediment surface, diel changes can significantly impact nutrient flux across the SWI.